Experience has indicated that by far the most difficult problem encountered is to achieve a free smooth movement of the spray gun, but this can be interefered with if the spray gun varies its speed of movement as it is guided for movement along tracks.
If a track comprises a number of different track members as in our aforesaid patent, it is difficult to achieve an accurate gauging between the surfaces which are engaged by the carriage rollers, and for that reason the cost is high. If however, a single track is used, for example a length of square tubing, there is not sufficient accuracy of surface to avoid irregular movement of a carriage if the rollers are constructed in accordance with known art (for example V groove rollers) but even if the surfaces are machined and V groove rollers are used, uneven movement can still result due to the fact that the outer surface of one roller groove may temporarily engage a portion of the track surface, while the inner surface may frequently engage another portion but will be rotating at a different speed from the outer surface so that a moment is imparted to the spray gun which has the same effect as varying the speed of travel.
The main object of the invention is therefore to provide improvements whereby the carriage movement of a spray gun reciprocating device will be very smooth, and this is primarily effected by utilising a pair of independently rotating rollers of each of a plurality of roller assemblies on a carriage with track engaging surfaces converging towards one another.
If use is made of the "air over oil" fluid to operate a piston/cylinder assembly for moving a carriage, with the end of the travel when the reciprocating device reverses, there is a large volume of air to be quickly discharged and this can result in "icing-up" of the air valve. This in turn will reduce the rate of discharge and cause a slow reversal of movement of the carriage which again will interfere with smooth operation of a spray gun, and in order to reduce this problem in this invention there is provided an expansion chamber connected to the air valves so that discharge air exhausts firstly into the expansion chamber before being released.